FORMER PM Kevin Rudd today became the new champion of gay rights by calling on Opposition Leader Tony Abbott to "have the decency" to give his MPs a conscience vote on same sex marriage.

And he urged holdout politicians to change their minds on the issue ahead of a June 6 vote in Parliament.

His call comes after he last night revealed his support behind same sex marriage in a blog post on his website.

"I have come to the conclusion that church and state can have different positions and practices on the question of same sex marriage," Mr Rudd wrote.

"I believe the secular Australian state should be able to recognise same sex marriage."

Today he said it was "time for some civility” in the national debate on this.

"We seem to have missiles being thrown in each direction, either from Christian fundamentalists in one direction or from various parts of the gay lobby in the other direction,” he said.

"I think the mood of the Australian nation is: How do we actually come together on this?"

He directed a message to Mr Abbott ahead of the June 6 vote: "Can I just suggest this? Just have the decency to extend the conscience vote to everybody.

He told reporters in Brisbane: "The Labor Party offers a conscience vote on these questions. Mr Abbott does not permit that of the Liberal Party.

"I don't think that is the proper way that this matter, which affects people deeply and personally, and is complex ethically...(it) is jot one where a party discipline should be imposed."

If Mr Abbott did not free up his MPs the prospects of a national plebiscite on the issue should be looked at "as the best way to resolve that".

Liberal MPs in theory have a free vote but front benchers are bound to the policy taken to the 2010 election to oppose gay marriage.

In practice the restriction also is obeyed by many back benchers.

Prime Minister Julia Gillard also opposes marriage equality and Mr Rudd said she no doubt had arrived at that position "as a product of her own conscience and her own deliberations".

She says she has not changed her stance on gay marriage, despite Kevin Rudd's backflip.

"I've made my position clear. I've also made it clear that I don't ask any Labor member to vote with me. What I ask them to do is to think deeply and to vote in accordance with their conscience," she said.

"Obviously, individual Labor members, including Kevin Rudd will use that freedom to reflect deeply and to form their own views about same-sex marriage.

"Every member is free to come to Parliament and vote in accordance with their conscience."

Mr Rudd added he had to explain to his voters in the Brisbane seat of Griffith why he had made his decision, and in his own words, on the eve of the expected June 6 House of Representatives vote.

He had waited "a bit of a lull" in political activity because he did not want to distract from Government business.

"For me to have said anything prior to the Budget would have been wrong because it would have taken focus off the Budget," Mr Rudd told reporters.

"And immediately after the Budget when you are dealing with the cell of the Budget by Government ministers around the country, I think that would have been wrong as well.”

Mr Rudd acknowledged his "coming out" on the issue was controversial was an instance of being true to his personal decision.

Mr Rudd said that the move had come after a lengthy period of reflection on the issue, including conversations with people of faith such as a "capital G God botherer", who had confessed he was gay and asked whether Mr Rudd had changed his position on same sex marriage.

"Some will ask why I am saying all this now. For me, this issue has been a difficult personal journey, as I have read much, and talked now with many people, and of late for the first time in a long time I have had the time to do both," he wrote.

"I have long resisted going with the growing tide of public opinion just for the sake of it. Those who know me well know that I have tried in good conscience to deal with the ethical fundamentals of the issue and reach an ethical conclusion."

Mr Rudd said his political enemies would interpret his announcement as a political manouevre.

"My opponents both within and beyond the Labor Party, will read all sorts of political significances into this, That's a matter for them. There is no such thing as perfect timing to go public on issues such as this."

Mr Rudd said he would not be taking any leadership position on the issue nationally.

Ms Gillard backed Mr Rudd's call for a Coalition conscience vote and called it a "step forward" for Mr Abbott.

The Prime Minister gave no indication she, too, had changed her mind but acknowledged that "inevitably and over timer" the numbers in Parliament could swing behind marriage equality.

Ms Gillard said that as Prime Minister she had made sure the Labor Party had a free vote, and pointed to conservative leaders overseas who had done the same for their members.

Mr Rudd's decision received a wave of support on social media with independent member for Sydney Alexander Greenwich posting on Twitter it was an "Obama-style" decision.

"Great news that @KRuddMP will be among key ALP MPs to vote for #marriageequality on June 6. Thank you @adambandt for putting your bill up".

"I think any observer would say there's an underlying trend as members of parliament give it more consideration and listen to the arguments to legalising same-sex marriage," Senator Carr told ABC radio on Tuesday.

He also acknowledged there was "considerable support" for legalising same-sex marriage among coalition MPs.

But Senior Liberal George Brandis said Mr Rudd's comments were more about personal ambitions than about the issue of same-sex marriage.

"It's not about same-sex marriage,'' he told Sky News.

"What it tells you is that Kevin Rudd has not given up, Kevin Rudd is at it again.''

Senator Brandis said the former prime minister was continuing to "dance this dance'' in front of the media for as long as he possibly could because his caucus supporters had not given up hope of him returning as Labor leader.

Marriage equality advocates have swiftly congratulated Mr Rudd for his change of mind.

"If Mr Rudd can show the maturity and integrity to compassionately evolve on this issue, so can the Prime Minister and Leader of the Opposition,'' Australian Marriage Equality national director Rodney Croome said in a statement.

Many Christians would not agree with his stance, he said, but he had never accepted the argument from some Christians that homosexuality was abnormal. "People do not choose to be gay," he said.

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